applet/pager: Divide window size by devicePixelRatio on X11 when it's >1

KWindowInfoPrivateX11::frameGeometry() returns the true geometry of
a window, so dividing the window size by devicePixelRatio is needed to
get the correct size when it's greater than 1.

BUG: 446627
FIXED-IN: 5.24.2


(cherry picked from commit 16d44ae2)
2 jobs for Plasma/5.24 in 3 minutes and 58 seconds (queued for 2 seconds)